Method for agglomerating and precipitating particles

ABSTRACT

An agglomeration and precipitation assembly which includes an agglomeration device in combination with a precipitation device. The agglomeration device is intended to receive a turbulent gas stream containing fine suspended particles and to discharge the stream in which the major part of the fine particles have agglomerated in the form of larger particles. The precipitation device is provided downstream of the agglomeration device to receive the stream coming from the agglomeration device and to separate the larger particles from the gas stream. The fine particles can be separated from a turbulent gas stream by first agglomerating the fine particles in the form of larger particles, and then separating the larger particles by precipitation.

This is a divisional application of U.S. Ser. No. 08/930,282, filed Sep.30, 1997, now U.S. Pat. No. 6,007,593, which is a 371 of PCT/FR97/00164,Jan. 28, 1997.

FIELD OF THE INVENTION

The present invention relates to improvements carried out to particleprecipitation devices and to particle agglomeration devices intended foruse with particle precipitation devices, and to the combination of aparticle agglomeration device and of a particle precipitation device.The invention relates both to the device and to the method implemented.

BACKGROUND OF THE INVENTION

U.S. patent applications Ser. Nos. 08/290,883 and 08/406,393, the secondbeing a continuation in part to the first one, filed on Aug. 18, 1994,now abandoned and Mar. 20, 1995 now abandoned (see also British patentNo. 2,264,655 B published on May 24, 1995, British patent applicationNo. 94/07,441.6 filed on Apr. 14, 1994, published international patentapplication WO 93/15,822 and international patent application WO95/00,489) respectively disclose a certain number of forms of particleprecipitation devices intended for separation of particles carried alongin a turbulent gas stream (generally, but not necessarily, air).Reference is made to these documents in the description hereafter. Theparticles can be solid or liquid.

In the specific description hereunder, one of the aspects of the presentinvention is notably explained for separation of an oil fog contained ina turbulent air stream, i.e. ultra-fine suspended oil particles,typically droplets of a size below 0.5 micron. Such oil-laden airstreams are encountered in the form of emissions of industrial machinesworking at high speeds.

However, it can be noted that this example of the present inventionrelative to the separation of oil droplets contained in an air stream isnot limitative within the scope of the present invention since theinvention also applies to the separation (precipitation) of othersuspended liquid or solid particles, for example dust, vapour or smoke.Such a separation can be performed by one of the precipitation devicesdisclosed in the aforementioned patent applications and patent, by thevariant of the previously disclosed precipitation devices described inthe present document or by any other form of precipitation device suchas an electrostatic precipitation device capable of performing thedesired separation of particles contained in a gas stream.

It has been discovered that the efficiency of a particle precipitationdevice is substantially higher if the particles have a certain minimumsize. Separation of particles with a size below one micron by means of aprecipitation device is less effective or may require a costlierequipment. For example, a suitable separation of particles having a sizebelow one micron can require a multi-stage precipitation device or aprecipitation device of a disproportionate length, which will lead to anunsatisfactory result or require setting of an excessively costlyequipment.

One of the objects of the present invention is to avoid thesedifficulties by proposing a method and a device intended to increase thesize of the particles contained in the gas stream before they enter theprecipitation device.

This objective is reached through agglomeration of the particles priorto their entering the precipitation device. Although the agglomerationtechnique described here is particularly effective when the particlesinitially occur in the form of a fog, it is also applicable to particlesexhibiting the form of dust, vapour or smoke. Furthermore, thistechnique is not limited to particles whose initial size is below onemicron. Nearly any size increase of the particles entering theprecipitation device is advantageous. Therefore, when it is said herethat this technique is aimed at agglomeration of "fine" particles, allthe particles that are too small to be subjected to a fast and effectiveseparation in a precipitation device, including the ultra-fine particlespresent in a fog, should also be included therein.

SUMMARY OF THE INVENTION

The present invention can thus be partly defined as a method ofseparating fine particles from a turbulent gas stream during which saidfine particles are first agglomerated in the form of larger particles,then said larger particles are separated from the gas stream byprecipitation.

The agglomeration stage can consist of a method of agglomerating fineparticles suspended in a turbulent gas stream wherein the gas stream ispassed successively through a series of filters so as to cause some ofsaid particles to collide with the solid parts of each of the filters inorder that they agglomerate in the form of larger particles. A largepart of these larger particles is re-entrained in the gas stream whilepart thereof falls from the filters prior to being collected.

Preferably, the stream is passed successively through at least tenfilters or through at least thirty filters.

The stream with the re-entrained larger particles is advantageouslypassed into a particle precipitation device.

According to one of the aspects thereof, the invention can be defined asthe combination of (a) an agglomeration device intended to receive aturbulent gas stream containing fine suspended particles and todischarge the stream in which the major part of said fine particles hasagglomerated in the form of larger particles and of (b) a precipitationdevice intended to receive the gas stream leaving the agglomerationdevice and to separate the larger particles from the gas stream.

The agglomeration device used in this combination can comprise a pipeprovided with an opening intended to receive the gas stream and with anoutlet intended for discharge of the stream, as well as a series offilters placed substantially parallel in the pipe and spaced out alongthe pipe between the inlet and the outlet, each filter extending acrossthe pipe generally crosswise with respect to the gas stream, so that allof the stream flows substantially through all the filters successively,each filter comprising solid parts distributed along the pipe andintended to be collided by a certain number of said particles, as wellas holes distributed along the pipe in order to allow passage of thestream.

The agglomeration device can also comprise means intended for dischargeof the agglomerated particles that are not re-entrained in the gasstream and fall onto the bottom of the plant.

According to the invention, the solid parts of each filter occupy asurface area that is less than the major part of the section of thepipe.

According to an embodiment of the invention, each filter comprises setsof parallel strands separated from one another and extending crosswisein relation to one another so as to form a meshed structure, thesestrands forming said solid parts, and spaces between the strands formingsaid holes; the ratio of the distance between the strands to thethickness of the strands approximately ranges between ten and five.

According to another embodiment of the invention, each filter comprisesa plate provided with holes.

Besides, the spacing of the filters with respect to one another in thedirection of flow of the stream must not be less than about fivemillimeters.

According to another embodiment of the invention, the filters consist ofa continuous meshed material extending successively over transverse rodssituated in the upper part and in the lower part of the pipe.

The precipitation device according to the invention can have the form ofany one of the devices described in the aforementioned patentapplications and patent; it may also be another precipitation devicecapable of separating particles from a gas stream.

The precipitation device can thus comprise at least one non-obstructedchannel intended to convey the turbulent stream, and a series of objectsextending along at least one side of each channel, said objects beingpositioned at close intervals in the direction of flow so as to definewith each other spaces which swirls coming from each channel enter,which leads to the accumulation of particles on the surface of saidobjects after the swirls have declined. According to an aspect of thepresent invention, the objects on whose surface the particles accumulateconsist of at least one corrugated plate.

The folds of each of said corrugated plates have a depth that is greaterthan the pitch between the folds.

Said depth is thus approximately four times as great as said pitch.

Preferably, each of said corrugated plates is positioned substantiallyvertically so as to allow the particles accumulated on the surfacethereof to fall to the bottom of a casing containing said plate(s) anddefining thus said channel(s).

The bottom of the casing is thus inclined to the horizontal in order tofavour the flow of the particles fallen from the surface of thecorrugated plate(s) towards an end of said bottom, then into an externalcollector.

The particles can be liquid and the external collector then comprises aliquid trap.

The particles can be solid and the external collector then comprises ahopper.

The precipitation device can also comprise a cup extending over thebottom of the casing and which contains the lower parts of saidcorrugated plates, characterized in that a small free space is providedbetween the lower ends of said corrugated plates and the bottom of thecasing in order to collect the solid particles fallen from the surfaceof the plates and to guide them below said corrugated plates through anopening provided in the cup so as to drive them towards a particledischarge slot situated at the end of the casing bottom.

A vibrator intended to favour transfer of the solid particles to theexternal collector can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of the layout of an agglomeration device and ofa precipitation device according to the invention,

FIG. 2 is a fragmentary cutaway side view of an agglomeration deviceaccording to an embodiment of the present invention,

FIG. 3 is a fragmentary cutaway top view of a part of the agglomerationdevice of FIG. 2,

FIG. 4 is a view along the line 4--4 of FIG. 2,

FIG. 4A is a fragmentary view of a variant of FIG. 4,

FIG. 5 is a fragmentary cutaway perspective view of a precipitationdevice according to another embodiment of the invention,

FIG. 6 is a cutaway side elevation of the precipitation device of FIG.5,

FIG. 7 is a cutaway bottom view of the precipitation device of FIGS. 5and 6, and

FIG. 8 is an enlarged fragment of a collecting element in theprecipitation device according to FIGS. 5, 6 or 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The particle precipitation device 10 shown in FIGS. 1 and 2 can be anyone of the precipitation devices described in the aforementioned patentor patent applications, and it can be more particularly one of thosepresented in U.S. patent application Ser. No. 08/406,393 or the variantthereof described hereunder in connection with FIGS. 5 to 8. It can alsobe any precipitation device capable of separating solid or liquidparticles from a turbulent air stream 12 drawn into the system by a fan11 or by any other means. The fan comprises a motor 11A and an outlet11B.

According to the invention, an agglomeration device 13 is situatedupstream from precipitation device 10 in relation to the direction offlow of the gas. The purpose of device 13 is to increase the size of theparticles carried along or suspended in air stream 12 so that thecollecting surfaces of precipitation device 10 can separate them moreeffectively from the stream.

The system presented in FIG. 1 was designed to separate liquid particlesfrom a gas stream, a fog for example, which is the reason why it isinclined to the horizontal and provided with a liquid trap 20. Theliquid accumulated in the system (mainly, but not exclusively, at thebottom of precipitation device 10) flows out at the bottom ofprecipitation device 10 and of agglomeration device 13, enters a drain19 and finally a trap 20 serving as an external liquid collector whilepreventing the air from being drawn into the system in this place.Typical liquid levels in drain 19 and in trap 20 are shown in FIG. 1.

When the system is designed for separation of solid particles, dust forexample, the latter characteristics are modified, as described hereunderin connection with FIGS. 6 and 7.

As may be seen in FIGS. 2 and 3, agglomeration device 13 can consist ofa tubular pipe 14 of rectangular section (in a specific example, 34 cmlong, 46 cm wide and 30 cm high) forming a tunnel between an openingconnected to the gas stream inlet and an outlet connected toprecipitation device 10. A series of filters 15 spaced out with respectto one another is placed in this pipe 14, each of these filtersextending fully across the pipe in the two directions perpendicular toair stream 12 so that, theoretically, all of the air stream must flowsuccessively through all the filters 15. In practice, a small amount ofthe stream can bypass said filters.

As shown in FIG. 4, which is not true to scale, a typical filter 15consists of two sets of transverse strands 16 forming a meshed network;these strands can be made of a suitable material such as polyester,glass fiber or metal. In a typical example, each strand 16 isapproximately 1 mm thick, the spacing of the strands being of the orderof 5 mm. A suitable method of forming this assembly when filters 15 aremade of a flexible material consists in using a very long strip of thismaterial and in passing it successively over the lower 17 and upper 18rods that extend across pipe 14. The diameter of these rods willdetermine the spacing of filters 15 which can approximately rangebetween 5 mm and 1 cm. If the filters are much closer to one another,they will not fulfil their purpose fully, as described hereunder,because they will not totally work as separate filters. If they are morespaced out, although they will work effectively, a device ofdisproportionate length will be obtained.

When the turbulent air stream 12 that is assumed to convey oil fogparticles of a size below one micron enters the inlet port ofagglomeration device 13, it has been experimentally discovered that asmall part of these particles separates off from the portions of the airstream flowing on each side of each strand 16, the particles thusseparated colliding directly with the strands. On each filter 15, only asmall fraction of the incoming fog particles collides with the strands,since most of the particles flow freely with the air stream through theholes between the strands. If one considers that a fraction (y) of theincoming fog particles collides with the solid parts (strands) of thefirst filter, the remaining fraction (1-y) will flow through the holes.The fog particles that have flowed through the holes with the air streamwill be mixed because of the turbulent flow and will have asubstantially uniform distribution before they reach the second filter.Furthermore, if need be, the strands can be positioned in staggered rowsbetween adjacent filters in order to guarantee the presence of strandsdirectly on the route of the particles that have flowed through theholes of the previous filter, with the air stream. When reaching thesecond filter, the same fraction (y) of the remaining fraction (1-y)will collide with the strands. The remaining fraction (transmittedthrough the holes) after the second filter is thus (1-y)-y(1-y)=(1-y)².After flowing through n filters, the fraction of the initial oil fogparticles remaining in the air stream will be (1-y)^(n). 0.04 is atypical value for y, i.e. 4%. If n=60, for example, the fraction ofparticles remaining after the stream has flowed through the last filterof the series will be 0.96 to the power 60, which approximatelycorresponds to 0.09. A fraction of about 9% of the initial fog particleswill thus remain in the air stream leaving the agglomeration device,whereas about 91% have collided with one or the other filter.

After impact, most of the substances forming the particles that havecollided with the filters tend to be re-entrained in the air stream.However, it has been observed that these re-entrained substancesconsisted of new particles, larger than the initial particles. In otherwords, the initial fine particles have agglomerated to give largerparticles. Some of these agglomerated particles remain in theagglomeration device and fall to the bottom of the plant, thus forming aliquid that flows into trap 20 and thus joins all of the particles thathave been collected. To obtain this result, it is important that the gasstream flowing through the agglomeration device exhibits a turbulentflow.

Within the scope of an experiment carried out with an oil fog producedby a sprayer, it has been measured that the size of about 80% by weightof the fog particles entering the agglomeration device was below 0.5micron. When this fog flowed directly into a 1-meter long precipitationdevice (when the agglomeration device had been removed), only 40% byweight of the particles were separated from the air stream. However,when the agglomeration device was situated between the incoming fog andthe same precipitation device, the latter separated approximately 93% byweight of the fog particles contained in the air stream. Thisperformance (93% recovery) could have been theoretically obtained with aprecipitation device alone (without an agglomeration device) if thelength of the precipitation device had been extended to five meters.Whereas the agglomeration device used alone only allows to collect apoor amount of particles of a size below one micron, the synergism ofthe phenomena occuring within the associated agglomeration andprecipitation devices allows to obtain a method providing the highperformances required for separation of fine particles without requiringimplementation of a longer precipitation device.

In order to demonstrate this synergism, it is assumed that particleagglomeration device 13 collects the oil fog with an overall fractionalefficiency (a) and that the precipitation device collects the same fogwith an overall fractional efficiency (b). In the absence of synergism,the fractional efficiency E of the combined agglomerationdevice+precipitation device system would be E=1-(1-a) (1-b). As we haveseen, the real fractional efficiency E' of the combined agglomerationdevice+precipitation device system is considerably greater than E. i.e.E'>>E. This does not only show the existence of a synergism, but it alsoshows the physical cause thereof, i.e. the fog flowing from theagglomeration device and entering the precipitation device is not thesame fog as the one which entered the agglomeration device; it is a fogmade up of particles of a considerably larger size, which is collectedby the precipitation device with a much higher fractional efficiencyb'>b than the one that might have been observed for the initial fog.Thus, 1-(1-a) (1-b')>>1-(1-a) (1-b). It has been determined that 80% byweight of the initial fog particles have a size below 0.5 micron,whereas the particles that constitute the fog flowing out of theagglomeration device have an average size of about 4 microns. Themeasured efficiencies are the following E'=0.93, b'=0.9, b=0.4 anda=0.3, whence E=0.58. It is therefore obvious that E'>>E.

In the aforementioned mathematical examples, it was assumed that 60filters were used, 57 filters having actually been used during theexperiment described above. Selection of the number of filters will be acompromise between an improvement in the performances (more filters) andin economy (less filters). In cases where a certain decrease inperformances is allowable, or if the incoming particles have a sizeabove one micron while being fine particles in the sense that they aretoo small to be separated directly in the precipitation device, it ispossible to use a more limited number of filters. Normally, this numberwill preferably not be below 30, but it can be brought down to 10, oreven less, when lower performances are allowable or when the value of ycan be increased, or else when the device must perform separation of afog containing droplets of a substantially larger size from thebeginning. There may thus be circumstances where a relatively limitednumber of filters can be effective. There is no maximum number, althougha number above 100 would normally not be very profitable in relation tothe advantage that would be gained therefrom. Preferably, the number offilters will thus normally range between 30 and 80.

Although each filter 15 has been described so far in the form of ameshed structure consisting of strands perpendicular with respect toeach other, it is also possible to use another structure such as aperforated plate producing the same effect, i.e. providing a largenumber of solid parts intended to be collided by certain particles,while leaving spaces intended to allow passage of the gas stream and ofthe remaining particles carried along FIG. 4A illustrates part of such aplate. Although implementation of a perforated plate may lead to anincrease in the head loss undergone by the gas stream, it can alsofavour an increase of the value of y and thus allow to reduce the numberof filters required, which would have a positive effect on the headloss.

It should be noted that the term "filter" used in the claims hereafterapplies not only to meshed structures, but also to non-meshed structuressuch as the perforated plate 16A shown in FIG. 4A, provided that thelatter fulfils a similar function by being provided with a surface onwhich solid parts collided by the particles and free spaces allowingpassage of the gas stream are distributed. In order to minimize headlosses, the surface area consisting of solid parts will normallyrepresent less than 50% of the total section of the pipe.

Several long-time tests have been carried out with an oil fog. After thetests were completed, it has been observed that the filters of theagglomeration device and the particle collecting elements in theprecipitation device were impregnated with oil. The flow rate was 1000m³ per hour and the velocity of the air stream was seven meters persecond. A very satisfactory separation of the oil droplets was observed,as well as an allowable head loss of only five centimeters of watercolumn.

The filters are preferably positioned vertically, the gas stream flowinghorizontally. However, these conditions are not rigid and it is possibleto deviate therefrom while allowing effective operation of theagglomeration device. The inclination of the system allowing theparticles collected in trap 20 to flow out will not exhibit a totallyhorizontal positioning and, as mentioned below, the degree ofinclination can be increased, for example to 15°, when solid particlesare to be collected. There would normally be no advantage in changingthe orientation of the flow of a gas stream, which is generallyhorizontal, and that of the filters, which are generally positionedvertically.

FIGS. 5 to 8 show details of the parts of the precipitation device thathave been modified in relation to the constructions described in theaforementioned patent and patent applications. However, the theory onwhich the particle separation performances of the variant shown in FIGS.5 to 8 are based basically remains the same as that applied in thispatent and in these applications.

In FIGS. 5 to 8, precipitation device 10 is provided with a casing 21forming a tunnel extending from an opening receiving the gas streamflowing from the agglomeration device or coming directly from an inletport, if the use of an agglomeration device is not necessary on accountof the relatively large size of the particles carried along, to anoutlet connected to fan (11).

In order to give the most exhaustive possible description, theprecipitation device is shown in FIGS. 5 to 8 in the form of a deviceintended for separation of solid particles, without an associatedagglomeration device 13. The basic principle of the construction cannevertheless also be applied to the separation of liquid particles,provided that a suitable liquid recovery system such as the dischargechannels and trap 20 is associated therewith as a substitute for thedust recovery system shown in FIGS. 5 to 8.

The collecting elements extending along casing 21 exhibit the shape ofcorrugated plates 22, preferably made of metal. Corrugated plates 22extend from the upper part of casing 21 to the vicinity of the lowerpart of the casing, thus leaving a free space allowing passage of thecollected dust through an opening 30 and a slot 25 prior to entering astorage hopper 26. Only a first corrugated plate 22 is shown in FIG. 5for clarity reasons. In practice, there will be a plurality of suchplates positioned side by side, for example the three shown in FIGS. 6and 7, placed in casing 21 and spaced out with respect to one another soas to form channels 23 allowing passage of the gas stream, positionedbetween adjacent plates and between the plates and the casing. As thisembodiment of the invention is designed for dust recovery and since dustdoes not flow as easily as a liquid, casing 21 has a greater inclinationof at least 15° for example to the horizontal 29, and it is connected toa vibrator 28 which causes the dust to fall onto the bottom. When thedust collected by corrugated plates 22 falls to the bottom of casing 21,part thereof might tend to disperse in the open channels 23 if it wasnot held back and it would be re-entrained in the gas stream. To preventthis, the lower parts of plates 22 are enclosed within cups 24. At thelower end (inlet) of casing 21, these cups 24 are provided with anopening 30 communicating with a slot 25 that extends across the bottomof the casing and communicates with a hopper 26 (not shown in FIG. 5 forclarity reasons) which serves as an external collector intended for dustrecovery and transportation. When an agglomeration device is used, thelatter can be advantageously housed in the same casing 21, or in acasing having the same section as casing 21, as in the case of therecovery of a fog shown in FIG. 1. In this case, the inlet of theagglomeration device comprises a slot 25 rather than a dust dischargeport. For dust recovery, a free space is also provided between thefilters and the bottom of the agglomeration device. A deflector 27 isprovided at the upper end (outlet) of casing 21 in order to cause thegas stream to leave the casing at a level situated above the bottom andtherefore to minimize any tendency towards re-entrainment of the dustfallen to the bottom.

In order to minimize re-entrainment of dust particles separated from thegas stream by the corrugated plates, but which have not fallen into thecups yet, the folds of plates 22 must be close-spaced, i.e. form anglesof low value. In other words, the depth of each fold in the direction d(FIG. 8) must be substantially greater than the pitch p. A d/p ratio ofthe order of four would be suitable. Although this ratio may be changedaccording to circumstances, it will be maintained at a valuesubstantially greater than one in order to obtain the best possibleperformances.

To sum up, the performances of a precipitation device intended forseparation of liquid or solid particles suspended in a gas stream areimproved, when the particles are fine or ultra-fine, for example belowone micron, by processing the gas stream before it enters theprecipitation device with a view to the agglomeration of the fineparticles in the form of particles of a larger size. This result isobtained by passing the gas stream successively through a series offilters. Some particles carried along in the gas stream collide with thesolid parts of each filter and agglomerate during the process. A largepart of the agglomerated particles is then re-entrained in the gasstream and flows through the precipitation device. As only a lowpercentage of particles collides with each filter, it is generallypreferred to use a relatively large number of filters, for example atleast 30. An improved embodiment of the precipitation device usescorrugated plates forming the surfaces on which the particlesaccumulate.

I claim:
 1. A method for separation of fine particles from a turbulentgas stream, comprising the steps of first agglomerating said fineparticles in the form of larger particles by successively passing thegas stream through a series of filters in order to cause part of theparticles to collide with solid parts of each filter so that part of theparticles agglomerate and form larger particles, most of said largerparticles being re-entrained in the gas stream, and then separating saidlarger particles by precipitation, wherein the step of separating saidlarger particles by precipitation comprises passing the gas streamhaving said larger particles reintrained therein from the series offilters through a precipitation device comprising at least onenon-obstructed channel intended to convey the stream in a turbulent flowand a series of objects extending along at least one side of eachchannel, said objects being positioned at close intervals in thedirection of flow so as to define with each other spaces which swirlscoming from each channel enter, which leads to an accumulation ofparticles at the surface of said objects after the swirl have declined,wherein said objects consist of at least one corrugated plate.
 2. Amethod as claimed in claim 1, wherein part of said larger particlesfalls from the filters.
 3. A method as claimed in claim 1, wherein thestream is passed successively through at least ten filters.
 4. A methodas claimed in claim 1, wherein the stream is passed successively throughat least thirty filters.
 5. A method as claimed in claim 1, wherein thefine particles consist of a fog.
 6. A method as claimed in claim 1,wherein the fine particles consist of dust, vapor or smoke.
 7. A methodas claimed in claim 1, wherein the folds of each of said at least onecorrugated plate have a depth that is greater than the pitch between thefolds.
 8. A method as claimed in claim 1, wherein said depth isapproximately four times as great as said pitch.
 9. A method as claimedin claim 1, wherein each of said at least one corrugated plate ispositioned substantially vertically so as to allow the particlesaccumulated at the surface thereof to fall to the bottom of a casingcontaining said plate(s) and thus defining said channel(s).
 10. A methodas claimed in claim 9, wherein the bottom of the casing is inclined tothe horizontal in order to favor the flow of the particles fallen fromthe surface of the corrugated plate(s) towards an end of said bottom,then into an external collector.
 11. A method as claimed in claim 10,wherein said particles are liquid and said external collector comprisesa liquid trap.
 12. A method as claimed in claim 10, wherein saidparticles are solid and said external collector comprises a hopper. 13.A method as claimed in claim 12, wherein a cup extends along the bottomof the casing and contains the lower parts of said corrugated plates,wherein a small free space is provided between the lower ends of saidcorrugated plates and the bottom of the casing in order to collect thesolid particles fallen from the surface of the plates and to guide thembelow said corrugated plates through an opening provided in the cup soas to drive them towards a particle discharge slot situated at the endof the casing bottom.
 14. A method as claimed in claim 13, furthercomprising vibrating the precipitation device to transfer the solidparticles towards the external collector.